In particular, yet not restrictive manner, the invention concerns a mechanical locking system for rectangular floor panels with long and short edges. It should be emphasized that long and short edges are only used to simplify the description. The panels could also be square. However, the invention is as well applicable to building panels in general. More particularly the invention relates to the type of mechanically locking systems which allow that all four edges of a panel could be locked to other panels by a single angling action preferably comprising a flexible or partly flexible tongue and/or displaceable tongue and/or a flexible locking strip in order to facilitate the installation of building panels.
A floor panel of this type is presented in WO2006/043893, which discloses a floor panel with a locking system comprising a locking element cooperating with a locking groove, for horizontal locking, and a flexible tongue cooperating with a tongue groove, for locking in a vertical direction. The flexible tongue bends in the horizontal plane during connection of the floor panels and makes it possible to install the panels by vertical folding or solely by vertical movement. By “vertical folding” is meant a connection of three panels where a first and second panel are in a connected state and where a single angling action of a new panel referred to as the “folding panel”, connects two perpendicular edges of the new panel, at the same time, to the first and second panel. Such a connection takes place for example when a long edge of the first panel in a first row is already connected to a long edge of a second panel in a second row. The new folding panel is then connected by angling to the long edge of the first panel in the first row. This specific type of angling action, which also connects the short edge of the new folding panel and second panel, is referred to as “vertical folding”. The short edges are gradually folded together and locked from one edge part to the other as scissors when the panel is angled down to the subfloor. It is also possible to connect two panels by lowering a whole panel solely by vertical movement against another panel. This specific type of locking is referred to as “vertical locking” A first row in a flooring system, which is designed to be locked with vertical folding, is often connected with a vertical locking where one short edge is pressed down vertically towards an another short edge. The other rows are connected with vertical folding. It is also possible to install a complete floor by connecting a row with vertical locking. The whole row is than connected to a previous installed row by angling.
Similar floor panels are further described in WO2003/016654, which discloses locking system comprising a tongue with a flexible tab. The tongue is extending and bending essentially in a vertical direction and the tip of the tab cooperates with a tongue groove for vertical locking.
Vertical locking and vertical folding of this type creates a separation pressure at the short edges when the flexible tongue or flexible parts of the tongue are displaced horizontally during the angling of the long edges. The inventor has analysed several types of floor panels and discovered that there is a considerable risk that the short edges could be pushed away from each other during installation and that a gap could occur between the edge portions of the short edges. Such a gap could prevent further installation and the floor panels will not be possible to connect. It could also cause serious damage to the locking system at the short edges. Pushing the floorboards sideways towards the short edges during installation could prevent the gap. Such an installation method is however complicated and difficult to use since three actions have to be combined and used simultaneously in connection with angling down of the long edges as described below.                a) The edges of a new floor panel has to be brought in contact with a first floor panel laying on the floor and the long edge of the new panel has to be pressed forward in angled position towards the first panel        b) The new panel has to be displaced sideways, in the pressed and angled up position, and pressed sideways against a short edge of a second panel laying on the floor in order to counteract the counter pressure of the tongue        c) The new panel must finally be angled down to the floor and the forward and sideways pressure must be maintained during the angling action.        
The inventor has discovered that separation and installation problems often occur when the panels have a small thickness and small compact locking systems on the long edges or when the panel core comprise a material with smooth surfaces such as high density fibreboard (HDF). Such problems could also occur when the panels are short or in connection with the installation of the first or last panel in each row since such installation is generally made with panels which are cut to a smaller length in order to adapt the floor to the wall position. Separation problems are of course extremely difficult to handle in any type of panels using locking systems with a strong flexible tongue that creates a substantial horizontal separation pressure during the vertical folding. Such strong tongues are very important in many applications where a high quality vertical connection is required and panels with such flexible tongues are very difficult to install with the known installation methods.
The invention aims to solve separation problems in flooring which is intended to be installed with vertical folding or vertical locking.
Definition of Some Terms
In the following text, the visible surface of the installed floor panel is called “front face”, while the opposite side of the floor panel, facing the sub floor, is called “rear face”. The edge between the front and rear face is called “joint edge”. By “horizontal plane” is meant a plane, which extends parallel to the outer part of the surface layer. Immediately juxtaposed upper parts of two adjacent joint edges of two joined floor panels together define a “vertical plane” perpendicular to the horizontal plane.
By “joint” or “locking system” are meant co acting connecting means, which connect the floor panels vertically and/or horizontally. By “mechanical locking system” is meant that joining can take place without glue. Mechanical locking systems can in many cases also be combined with gluing. By “integrated with” means formed in one piece with the panel or factory connected to the panel.
By a “flexible tongue” is meant a separate tongue which has a length direction along the joint edges and which is forming a part of the vertical locking system and could be displaced at least partly horizontally during locking. The whole tongue could for example be bendable or it could have flexible and resilient parts that can be bend to a locked position or that could bent and spring back to its initial position.
By “angling” is meant a connection that occurs by a turning motion, during which an angular change occurs between two parts that are being connected, or disconnected. When angling relates to connection of two floor panels, the angular motion takes place with the upper parts of joint edges at least partly being in contact with each other, during at least part of the motion.
By an “angling locking system” is meant a mechanical locking system which could be connected vertically and horizontally with angling comprising a tongue and a grove that locks two adjacent edges in a vertical direction and a locking strip with a locking element in one edge of a panel called “strip panel” that cooperates with a locking groove on another edge of a panel called “grove panel” and locks the edges in a horizontal direction. The locking element and the locking groove have generally rounded guiding surfaces that guide the locking element into the locking groove and locking surfaces that locks and prevents horizontal separation between the edges.
With “installation angle” is meant the generally used angel between two panels which are in the initial stage of an angling installation when one panel is in an upwardly angled position and pressed with its upper edge against the upper edge of another panel laying flat on the sub floor. The installation angle is generally about 25 degrees and in this position there is only two contact points between the strip panel and the grove panel. In very special cases, where there may be more than two contact points between the connectors, the installation angle is higher then 25 degrees.
With “three point contact angle” is meant the angle between two floor panels during angling when there are at least three contact points between parts of the locking system.
With “contact angle” is meant the angle of the folding panel when the short edge of one panel is brought in the initial contact with the part of the flexible tongue which is intended to be displaced horizontally and which is active in the vertical locking at the short edges.
With “guiding angle” is meant the angle between two floor panels during angling when guiding surfaces of the locking element on the locking strip and/or on the locking groove are in contact with each other or with the upper part of the locking element or the lower part of the locking groove respectively. Guiding surfaces are often rounded or bevelled parts that during angling press the upper edges of the panels towards each other and facilitate the insertion of the locking element into the locking groove. Most locking systems on the market have a guiding angle of about 5 degrees.
With “locking angle” is meant the angle between two floor panels at a final stage of an angling action when the active locking surfaces on the locking element and the locking groove are in an initial contact with each other. Most locking systems have locking angles of about 3 degrees or lover.
With “friction angle” is meant the angle when a friction along long edges increase considerably during angling from an installation angle due to the fact that more than two contact points are active in an angling locking system and counteracts displacement along the long edges.
With “tongue pressure” is meant the pressure in N when a tongue is in a predetermined position. With “maximum tongue pressure” is meant the pressure of the tongue when it is in the inner position during vertical folding and with “tongue pre tension” is meant the tongue pressure in locked position when the tongue presses against a part of the tongue grove.